Frequency multiplier



y 1935. F. E. TERMAN 2,000,362

FREQUENCY MULTIPLIER Filed May 51, 1950 Alumna INVENTORT FPEDE ICK E. TERMAN. BY 56 ATTORNEY Patented May 7, 1935 UNITED STATES PATENT oF'FicE 8"Cla'ims.

,My invention relates, to frequency .multipliers or harmonic generators, forgenerating currents or voltages whose frequency-is adefinite,.-integral multiple of the frequencyof some-given source.

- Such generators are useful in producing currents of high frequency, and great frequency stability from a low frequency standards, such, .for -ex ample, as an oscillating crystal. They :are also of value where it is desired 'togenerate currents T of a plurality of frequencies'having definite specified relation "to each other.

Objects of myinventionare: First, to provide a frequency multipliercapable'oi generating relatively large amounts -of-power upon the higher harmonics of the input source; second, to providea generator of 'thevacuum tube type whose harmonic output is of the same order of magnitude as its fundamental frequency :input, so that successive frequency multiplications may be carried out without the use of intermediate amplifiers; third, 'to provide a frequency multiplier which may be used either to provide-a largecurrent output at some specified harmonic, or ito;.provide a current output which is rich in a multiplicity of harmonics; fourth, to provide 3/1181- monic amplifier in which large frequency :multiplication may be accomplished with a limited number of tubes; and.fifth,to provide a harmonic generator of the type described which is extremely stable in operation and simple to use.

wMy invention possesses numerous other objects and features of advantage, some of which, with the foregoing, will beset forth in the following description of my invention. It is .tobe understood that I -do not limit myself to this disclosure of species of my invention,.as,-I.may. adopt variant embodiments thereof within the scope of the claims. r l

Referring to thedrawing:

Figure 1 is a diagram showing cuit of the frequency multiplier.

Figure 2 is a diagram showing the circuit as used to produce currents having a rich harmonic spectrum.

Figure 3 is a diagram showing the circuit as modified to emphasize a selected harmonic frequency.

Figure 4 is a circuit diagram of a neutralized harmonic generator for delivering a maximum output of a selected harmonic. 1

Figure 5 is a circuit diagram showing a combination of cascaded frequency multipliers, for producing extremely high multiplications with alimitednumber'of tubes.

.The ordinary typenf harmonic generator :com-

the basic cir prises .a vacuum tube which is operated at a strong negative bias, preferably of suflicient-value wholly to suppress plate current 'during 'one-half of the input current cycle. A second type which is in general use comprises merely an overloaded 5 amplifier tube. The output current in either of these cases is an asymmetric wave, whichcom prises a considerable component of the fundamental or input frequency, relativelyla'rge proportions of the 2nd and 4th harmonics of this frequency, and diminishing quantities of' the higher harmonics. The harmonic output of a tube so operated is relatively limited as compared to; the output of the same tube operated as a distortionless amplifier, and in order to get s'ufli-' cient power from the device to operate -a suc-' ceeding'tube to produce -re-m11ltiplication, it is necessary either to utilize 'the lower harmonics, in which case a large number of multiplications maybe necessary, or to amplify the harmonic 20,

output between multiplications, which again necessitates the use of a relatively large number'of tubes.

In the frequency multiplierof my invention,

no excessive grid bias is used for producing distortion in the plate circuit. Instead, the tube is so operated that its input circuit absorbs power during-a portion of the cycle, and the necessary distortion is produced by the non-linearity of the input circuit. With the input circuit of the device thus arranged to absorb power, a distorting network is connected in the circuit in such a manner that the distorted input current wave produces a voltage drop across the input of the tube at thedesired harmonic frequency. Ahare monic voltage thus appears upon the grid or other control electrode of the tube, and is amplified by the tube to produce an amplified outputlpower. Either the distorting network, theoutput circuit of the tubepor both may be tuned to a specific harmonic frequency, resulting in a very large proportion of the output power of 'thetube be ingavailable at the harmonic frequency. When very large frequency multiplications are desired,

it'ispossible'tocascade two or moreof thetubes, and feed the harmonic output of one of these tubes back into the input of ,apreceding tube. Since the frequency fed'back is not the same as was-originally applied'to the first tube, :re-genoration and self-oscillation of the system do not 50.

take place, and the frequency fed back canbe passed thruia second distorting network, and the .same tubes thus used for re-multiplying the frequency. Extremely. large multiplications may be accomplished in this manner with very few tubes.

tortion, the latter The fundamental'circuit employed in my invention is shown in Figure 1, wherein a vacuum tube I is supplied with fundamental frequency by a suitable generator I l. The tube is biased by a battery or other potential source 12, the bias preferably being so arranged that, with the potential applied, the plate current of the tube is never completely cut off, while the grid or other control electrode is allowed to swing positive during a portion of the cycle. For most favorable operation the applied potential should be relatively large, approximately of the same order as is applied to the input circuit of the tube when it is operated as a distortionless amplifier, and sufficient to swing the grid potential positive during a portion of the cycle.- 7 I The tube is provided with the usual plate supply l3, feeding the plate thru a high impedance choke-coil IS. The plate supply is arranged in parallel with the load circuit, which comprises an impedance Z in'series with a blocking condenser ll.

A frequency-distorting network having an impedance Zg is connected in series with the source of supply frequency and the control electrode of the tube. This distorting network may, for various purposes, assume a wide variety of forms. A pure resistance is not suitable, since the effect of such an element is merely to limit the grid current without producing a corresponding change in voltage wave form. The desired characteristic 'of the network is that the grid current which flows thru it shall'produce in it a voltage drop which will cause a marked deformation of the voltage wave impressed upon the grid. Ob,-

viously, Z may be the impedance of the source ll if this varies the frequency, but in general it is more convenient to provide it as a separate element as will be shown and described in the succeeding figures, wherein the results of varying1 charactersof Z and Zp are discussed in do:

It should be noted that the distorting network, of itself, does not create harmonics; i. e., its impedance varies with frequency rather than with applied voltage or current. The harmonic generation is accomplished in the input circuit of the tube itself, and the function of the network is merely to provide a voltage drop which increases as the current drawn by the tube increases, thus accentuating greatly the harmonic voltage, as compared to the fundamental voltage. impressed on the grid of the tube. For this reason, in accordance with the usual practice of classifying distortions as phase distortion, harmonic or a linear distortion and frequency disterm is used to describe the network. r

In the circuit shown in Figure 2, Zg takes the form of a series inductance 20, while Zp is a simpleresistance 2|. Such an arrangement is desirable where it is wished to generate many harmonics of the same fundamental frequency. It will be seen that the inductance 20 offers an impedance to the input current to the tube which increases with the order of the harmonic. Therefore, not only does the input current of the tube vary in a non-sinusoidal manner with the volt age from the generator, but this non-sinusoidal current produces a voltage drop thru the inductancecoil which is impressed upon the grid of the tube, and which the tube amplifies to produce a non-sinusoidal voltage drop in its output circuit. Owing to the high impedance of the choke-coil l thiscurrent flows thru the conis desired the resistance 2| may be replaced by a tuned circuit as is shown in Figure 3. Where this arrangement is used the inductance 22 is "tuned -by means of the condenser 23 to the frequency of the particular harmonic which it is desired to accentuate. Under these circumstances the impedance of the tuned circuit to the harmonic frequency is very high, and relatively large currents are caused to circulate in the tuned circuit at the harmonic frequency. At other frequencies, the tuned circuit offers a very low impedance, very small circulating currents are set up, and the other harmonics which would otherwise be generated in the tube output are very largely suppressed. I

It is to be noted that the output efficiency ofthe tube improves if the condenser I1 is made sufiiciently small to have a relatively high impedance to the fundamental frequency. This de-. creases the current of fundamental frequency which the tube I0 is called upon to supply, and consequently increases the tube capacity at the harmonic frequency.

The harmonic being by definition of higher frequency than the fundamental, the condenser l1 offers relatively low impedance at this frequency. By this arrangement a relatively large voltage of the selected harmonic, relatively uncontaminated-by either the fundamental or other harmonics, may be taken from across the inductance 22.

Still greater eificiencies at a specific harmonic frequency may be obtained by the arrangement in Figure 4. Here the alternating source I l' is fed into the tube It) thru a distorting network comprising a parallel resonant circuit formed from an inductance 25 tuned to the harmonic frequency by a condenser 26. The control electrode is biased as before by a battery I2. In this case, the plate supply I 3 is arranged for series-feed thru a parallel resonant circuit 21 tuned to the desired harmonic frequency, in series with a parallel resonant circuit .28 which is tuned to the fundamental frequency.

The circuit 28 serves the same purpose as the small blocking condenser IT in the preceding figure, in. that it offers a high impedance to the fundamental frequency. .The circuit 27 carries oscillations at the harmonic frequency, and the harmonic voltage may be withdrawn from across it for whatever purpose it is desired.

' Secondarycoils 30 and 3| may be coupled to the two oscillating circuits and connectedthru neutralizing condensers 32 and 33 respectively to the grid of the tube ID, in order to prevent self-oscillation. This permits the tuned distorting circuit and the circuits 2'! to be resonated exactly to the desired harmonic frequency, and since the damping of "each of these circuits is small the oscillations tend to continue even during the period of minimum harmonic amplitude in the grid current, and hence tend to provide harmonic current or voltage in the outputcircuit of relatively constant amplitude. 1

By de-tu'ni'ng the circuits 21 and '28 slightly from exact resona-nceto the fundamental and harmonic frequencies, the neutralizing arrangement may be done away with, at the expense of small loss in amplitude and uniformity of harmonic output. The method of employing mysystem in cascade frequency ri-rultiplic'a-tion is shown in Figure 5. It is assumed for illustrative purposes that each frequency multiplication is by a factor of 6', altho almost any harmonic may be chosen, and successive multiplications may be by different factors. a

The source supplies current at the fundamental frequency thru the tuned distorting network 36' to the grid of the tube 37. A second distorting network 38 is also connected in series, but as it is tuned to a much higher frequency than the network 33 it offers relatively low impedance to this frequency, Assuming as above stated, that the circuit, 35 is tuned to the 6th harmonic of the source 35, the oscillating plate current of the tube will be passed thru the condenser 45, the resonant circuit 4! which is tuned to approximately the 6th harmonic frequency, and the second tuned circuit 42, tuned to a higher frequency.

The voltage across these two tuned circuits is applied to the grid of a second tube 43 thru another pair of distorting networks. Of these, the first network 45 is tuned to the 6th harmonic of the circuit 4| or the 36th harmonic of the fundamental frequency. The second is tuned to a still higher frequency.

The output circuit of the tube 43 is similar to that of the tube 31, except that the blocking condenser 41 is made sufliciently small to offer high impedance to the 6th harmonic of the source, which is the fundamental frequency applied to the tube 43. Of the two tuned circuits in the output of this second tube, circuit 48 is tuned to approximately the same frequency as network 45, and circuit 50 to approximately the same frequency as network 43.

A coil 5!, coupled to circuit 48, supplies a voltage thru the line 52 to a coil 53 which is coupled to a second coil 54 in series with the source 35. This coil impresses a high 36th harmonic voltage upon the grid of the tube 31 together with the fundamental frequency from the source 35. A condenser 55 may be provided to by-pass, around the source 35, the currents thus fed back.

The distorting network 38 is tuned to the 6th harmonic of this second impressed voltage upon the tube, i. e., to the 216th harmonic of the source.

The tube therefore operates as a harmonic amplifier at this frequency. In a similar manner the distorting network 46 in the input of the tube 43 again raises this frequency to the 6th harmonic, providing in the output of the tube, across the tuned circuit 50, a voltage of the 1,296th harmonic of the source, which is delivered to its point of use by the line 53.

The use of the 6th harmonic in the above example is illustrative merely. Had 8th harmonics been chosen thruout, the frequency multiplication would have been 8 or 4,096, since there are four frequency multiplications accomplished, each leading to a multiplication of 8 times. Thus a 1,000 cycle source of power may readily be used to generate frequencies well up in the radio broadcast band and even higher, with the use of a very limited number of tubes.

Certain clear distinctions betweenthe frequencymultipli'er of my invention and those which have heretofore been used should be mentinned here: A conventional frequency multiplier isuseful primarily in generating the lower har 1 monies, andits output of higher harmonics can not be materially increased even by tuning its output circu'itto the harmonic frequency; inthe frequency multiplier of my invention tuning eitherthe distorting network or the output circuit to the harmonic frequency results in a materially increased harmonic output. For best results, frequency multipliers of' conventional type should have low output impedance to the fundamental frequency, and this results in large dissipation of tube power upon this frequency; the generator of this disclosure gives its best performance when the tube' output offers high impedancexto the fundamental frequency, thereby reducing the du'tyuponthe tube and increasing its harmonic output.

The outstanding characteristics of the harmonic amplifier of my invention as opposed to those of conventional typelies' in its effectiveness in generating the very high harmonics. The form of the circuit shown in Figure? will deliver as much power upon the 50th harmonic as will the conventional type upon the 12th harmonic, while with the circuits shown in Figures 3 and 4, the efiiciencies upon high harmonics are a 1,000 or more times as great as with the usual type of device. The discrepancies are not as great on the lower harmonics, altho even here the advantage lies with the device herein described.

As a general rule, it may be stated that the frequency multiplier of my invention will give a harmonic output of substantially the same order as the tube used is capable of supplying as a distortionless amplifier.

Although it is usually desirable to use an output circuit having a high impedance to the fundamental frequency and to the multiplied frequency, this is not an essential. By making the output impedance low at these frequencies it is possible to combine in a single tube the functions of the frequency multiplier here described with the usual plate-distortion frequency doubler. By this method it is possible to obtain, say, the 6th harmonic by grid distortion as here described, and double this frequency by platedistortion, obtaining a 12th harmonic output.

I claim:

1. A frequency multiplier comprising the combination of an amplifying device the input impedance whereof varies with voltage and which supplies an output current substantially proportional to input potential, a source of alternating current at a potential surficient to cause changes in said input impedance, and an input circuit for said amplifying device comprising a network the impedance whereof varies with frequency connected in series with said source.

2. A frequency multiplier comprising the combination of an amplifying device the input impedance whereof varies with voltage and which supplies an output current substantially proportional to input potential, a source of alternating current at a potential sufficient to cause changes in said input impedance, and an input circuit connected in series with said source and said amplifying device comprising a network the impedance whereof is less at the fundamental frequency of said source than at a harmonic frequency thereof.

3. A frequency multiplier comprising the combination of an amplifying device the input impedance whereof varies with voltage,'a source of alternating current at a potential sufficient to cause changes in said input impedance, and an input circuit forsaidamplifying device comprising a parallel resonant circuit resonant to a frequency higher than the frequency of said source in series with said source and said amplifying device.

4. A frequency multiplier comprising an amplifying vacuum tube having input and output terminals, a frequency-distorting network connected to the input terminal of said tube, and an output circuit for said tube having high impedance to both the frequency to be multiplied and the desired multiple thereof as compared to its impedance to other frequencies.

5. A frequency multiplier comprising an amplifying vacuum tube having input and output terminals, a frequency-distorting network comprising an inductive element connected in series feeding relation with said input terminal, and

an output circuit comprising an impedance tuned substantially to the'desired multiplied frequency.

6. A frequency multiplier comprising an amplifying vacuum tube having input and output terminals, a frequency-distorting network comprising an inductive element connected in series with said input terminal, an output circuit comprising an impedance tuned substantially to the desired multiplied frequency, and means in said output circuit offering high impedance to the frequency to be multiplied.

7. A frequency multiplier comprising a vacuum tube amplifier having a control electrode biased to draw an appreciable current, and means connected to said control electrode for selectively limiting the current drawn thereby at the desired multiplied frequency.

8. The method of operating an amplifier tube to produce harmonics of an applied frequency in the output circuit thereof, which comprises the steps of applying said frequency to said tube at a mean potential such that the current drawn by said tube varies with the instantaneous potential thereof, limiting selectively the current drawn by said tube at the desired multiple frequency, and impressing the voltage drop produced'by the selective limitation of current on said tube for amplification.

FREDERICK E. TERMAN. 

